Distributions and extreme conditions

For many species, distributions are accounted for not so much by average temperatures as by occasional extremes, especially occasional lethal temperatures that preclude its existence. For instance, injury by frost is probably the single most important factor limiting plant distribution. To take one example: the saguaro cactus (Carnegiea gigantea) is liable to be killed when temperatures remain below freezing for 36 h, but if there is a daily thaw it is under no threat. In Arizona, the northern and eastern edges of the cactus' distribution correspond to a line joining places where on occasional days it fails to thaw. Thus, the saguaro is absent where there are occasionally lethal conditions - an individual need only be killed once.

Similarly, there is scarcely any crop you only die once that is grown on a large commercial scale in the climatic conditions of its wild ancestors, and it is well known that crop failures are often caused by extreme events, especially frosts and drought. For instance, the climatic limit to the geographic range for the production of coffee (Coffea arabica and C. robusta) is defined by the 13°C isotherm for the coldest month of the year. Much of the world's crop is produced in the highland microclimates of the Sao Paulo and Paraná districts of

Figure 2.14 (a) The northern limit of the distribution of the wild madder (Rubia peregrina) is closely correlated with the position of the January 4.5°C isotherm. (After Cox et al., 1976.) (b) A plot of places within the range of Tilia cordat (•), and outside its range (o) in the graphic space defined by the minimum temperature of the coldest month and the maximum temperature of the warmest month. (c) Margin of the geographic range of T. cordata in northern Europe defined by the straight line in (b). ((b, c) after Hintikka, 1963; from Hengeveld, 1990.)

Figure 2.14 (a) The northern limit of the distribution of the wild madder (Rubia peregrina) is closely correlated with the position of the January 4.5°C isotherm. (After Cox et al., 1976.) (b) A plot of places within the range of Tilia cordat (•), and outside its range (o) in the graphic space defined by the minimum temperature of the coldest month and the maximum temperature of the warmest month. (c) Margin of the geographic range of T. cordata in northern Europe defined by the straight line in (b). ((b, c) after Hintikka, 1963; from Hengeveld, 1990.)

Figure 2.15 The incidence of southern corn leaf blight (Helminthosporium maydis) on corn growing in rows at various distances from trees that shaded them. Wind-borne fungal diseases were responsible for most of this mortality (Harper, 1955). (From Lukens & Mullany, 1972.)

Row number from shading trees at edge of field

Brazil. Here, the average minimum temperature is 20°C, but occasionally cold winds and just a few hours of temperature close to freezing are sufficient to kill or severely damage the trees (and play havoc with world coffee prices).

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